Are you using a template that has a fullscreen option?
I find with my WP theme that the fullscreen template which I'm using auto scales down the images if the browser isn't expanded to full screen.

Even if the images aren't full screen images, they only appear razor sharp when the browser is expanded. Check out my site to fully understand. Click 'THE WORK', look at some of the photos with a small browser, then fully expand your browser and take a peek.